1. Field of the Invention
The present invention relates to a digital image converter and is particularly directed to a method for extending the range of radar recurrence frequencies which can be accepted by the converter, as well as means for carrying out said method.
2. Description of the Prior Art
The essential function of a digital image converter (or DIC) is conversion of an image produced in polar coordinates and having relatively slow refresh rate to a luminous television-type image permitting utilization in an illuminated environment. This slow-refresh rate image is usually a radar image but can also be derived from a sonar, an infrared sensor or an echographic system to be displayed on a screen which operates in the television mode.
In the case of radar, a conventional DIC usually comprises the following elements shown in FIG. 1.
A television display 5 on which radar data are displayed.
A radar interface circuit 1 which receives the video signals delivered by the radar. These video signals are constituted by a synchronizing signal (S.sub.y) indicating that a pulse is emitted by the radar and by the video output (.rho.) properly constituted by all the responses (echoes) to this pulse. This interface circuit is intended in particular to convert the incident video signal (analog signal) into a digital signal and to process the video signal, the processing details being different depending on the type of radar.
A device 2 for conversion of coordinates which receives the video signals delivered by the circuit 1. The radar data is delivered in polar coordinates and must be written on the television screen in cartesian coordinates. To this end, the device 2 also receives signals which are representative of the rotation of the radar antenna. The function of these rotation signals is to indicate at any moment the angular position of the radar beam (radar antenna) as this latter rotates in a uniform manner with respect to a reference direction which is usually north. The rotation signals consist of a north signal (N) which is a pulse delivered each time the radar beam passes through north and of a signal representing an angle increment (.epsilon.) and indicating that the beam has rotated through 1/n.sup.th of a revolution with respect to the preceding increment if n increments correspond to 360.degree.;
a memory 3 designated as an image memory and having a capacity adapted to the television standard employed i.e. must have a number of storage locations corresponding to the number of points (or pixels) of the image displayed on the television screen. Furthermore, the luminance of each image pixel is coded by means of a predetermined number of bits such as, for example, three bits permitting eight levels of luminosity for each pixel. Reading towards the television display and writing of the radar information delivered by the circuit 2 are asynchronous: the reading operation has priority and writing is stopped during a reading step.
Read-out circuits 4 for generating television synchronizing signals, for reading one or a number of memory locations and for digital-to-analog conversion of the read data in order to generate the television video signal for the display 5.
A persistence circuit 6 for producing a persistence effect for stored digital data in which no modification exists due to aging. This persistence effect is comparable with the effect produced on a persistence tube in which the brightness of a pixel begins to decay as soon as it is written.
In a system of this type, the process of writing in memory is as follows: the video output is constituted by the responses (echoes) to a pulse emitted by the radar and is produced at the frequency of repetition of transmitted pulses, usually designated by the acronym PRF (Pulse Repetition Frequency). It should be noted that the PRF is determined by the range of the radar. The radar range is the maximum distance at which an echo can be recorded, thus determining the time taken by the pulse to travel to the target and return to the radar (round-trip travel time) and consequently a repetition period or frequency. The video data delivered at the PRF will hereinafter be designated as recurrences. It should also be mentioned that the angle increments .epsilon. are usually delivered by the antenna coder in an asynchronous manner with respect to the recurrences. The angle .theta. designates the resultant angle between the radar beam at a given instant and the reference direction (north). The video information assigned to each of the n directions .theta. of the beam will hereinafter be designated as "radial". When the radar system delivers n angle increments, it is usually desired to display n "radicals". It is accordingly necessary to assign the different recurrences to different radials, that is one recurrence or a number of recurrences to each "radial". The different points or pixels of each radial are then written in the image memory at addresses computed in cartesian coordinates.
However, since the periodicity of the angle increments .epsilon. is given by the antenna and the PRF is determined separately by the range of the radar, it appears that, in some instances, these values may be such as to give rise to one of the following unfavourable configurations:
Either the frequency of the increments .epsilon. is too low with respect to the PRF. Among the recurrences received at the PRF, there are in that case recurrences which do not correspond to any value of .theta.. Accordingly, they are either not written in memory and lost or else they are grouped together beforehand (in a manner which may not be constant from one radial to the next or from one antenna revolution to the next) and written in memory. The disadvantage which results from this is an error in positioning of echoes. Further, that positioning error tends to vary from one revolution to another which consequently results in disturbed operation of the persistence.
Or the frequency of the increments .epsilon. is too high with respect to the PRF. In this case there are values of angle .theta. for which no corresponding radar information (recurrence) is available. Accordingly, there is no entry into image memory due to lack of information. This results in the disadvantage of image irregularity caused by lack of a radial from time to time as well as disturbed operation of the persistence.
In order to overcome these drawbacks, it is found necessary in practice to modify the PRF which in turn causes a limitation of the radar image.